This invention relates to electronic devices for receiving a plurality of radiated electromagnetic signals, filtering a selectable channel of frequencies from the received signals, and demodulating the signals of the selected filtered channel. More particularly, the invention relates to television receivers.
Television receivers of the prior art include a radio frequency (RF) section and an intermediate frequency (IF) section. The RF section includes RF filters which are tuned to coarsely filter a band of channels centered about a manually selected channel. The output of the RF filter couples to the input of an RF amplifier. Typically, total gain through the RF section is at least 20 to 30 dB. This gain increases the amplitude of signals within the selected channel and additionally makes the noise figure of the system essentially independent of subsequent elements in the receiver. The output of the RF amplifier couples to one input of a mixer, while a second input of the mixer receives mixing signals of a selectable frequency. The selectable frequency is generated such that the selected channel is frequency shifted to approximately 45 MHZ. The output of the mixer couples to a channel selection filter which provides a relatively high impedance path for frequencies outside of the selected channel, and a relatively low impedance path for signals inside the selected channel. Signals at the output of the channel selection filter are therefore primarily comprised of frequencies within the selected channel.
Each television channel contains audio information, video information, and frame synchronizing information. The output of the channel selection filter couples to an audio demodulator which separates the audio information from the selected channel; and the output of the audio demodulator couples to a speaker which generates audible sounds. Similarly, the output of the channel selection filter couples to a video processing unit which separates the video and frame synchronizing information from the selected channel; and the output of the video processor couples to a picture tube which converts the video and frame synchronizing information to pictures.
As described above, prior art television receivers insert at least 20 to 30 dB of gain in the RF section to achieve a low system noise figure. The system noise figure equals EQU NF.sub.1 +(NF.sub.2 -1/G.sub.1)+(NF.sub.2 -1/G.sub.1 G.sub.2)+(NF.sub.2 -1/G.sub.1 G.sub.2 G.sub.3)+ . . .
wherein NF.sub.i and G.sub.i are the noise figure and gain of the ith functional block in the system. Thus, inserting a large gain in the first functional blocks (i.e. the RF section) lowers the system noise figure by making it independent of the noise figure of subsequent circuitry.
However, a high gain RF section generates output terms which are proportional to its inputs cubed or raised to higher order odd powers. This is because the gain occurs before channel filtering, and signals in an undesired channel invariably become so large as to exceed the dynamic range of the circuits. In a television receiver, these odd power terms may generate interfering signals in the desire channel. For example, such interfering signals are generated when signals are present in channels on one side of the selected channel which are one and two channels removed from the selected channel. This phenomena is known as intermodulation distortion. Similarly, the cubic terms and higher order odd power terms generate interfering signals in the desired channel when a carrier with amplitude modulation is present in any one of the undesired channels. This phenomena is known as cross-modulation distortion. The frequencies which are generated in a desired channel as a result of intermodulation distortion or cross-modulation distortion cannot be separated from the information signals lying therein. Thus, as the magnitude of the interfering frequencies increases, perceptible picture distortion or sound distortion occurs.
By comparison, the disclosed invention has a unique architecture which achieves a low noise figure, low intermodulation distortion and low cross-modulation distortion simultaneously. In the preferred embodiment, the disclosed invention has a maximum gain in the channel selecting section which is less than 10 dB; that utilizes circuit elements therein which individually have low noise figures. The 10 dB gain in combination with the individual low noise figures yield a low system noise figure, while the low gain allows RF and IF circuit elements to operate within their dynamic range without generating high odd order output terms.
The architecture of the disclosed invention is also novel in that it has two IF frequencies about which the system gain is selectively distributed. In one preferred embodiment, a first mixer shifts the selected channel to about 330 MHz. The output of this mixer couples to the channel selection filter; and the output of the channel selection filter couples to a second mixer which frequency shifts the filtered selected channel to a second IF frequency of approximately 45 MHz. Most of the gain of the system (approximately 60 dB) is inserted at the lower IF frequency after the channel selection filter. Gain at the high IF frequency is small as pointed out above. As a result, several advantages of a high IF system - such as simplified filtering of image frequencies-are obtained, while feedback inherent in a high gain-high IF section is avoided.
The disclosed system is also simpler and potentially less expensive than prior art television receivers in that the low gain RF section makes feasible integration of a major portion of the receiver on a single semiconductor chip. Incorporated in the disclosure are two differently organized RF-IF sections which are suitable for semiconductor chip integration.
The intermodulation distortion and cross-modulation distortion in a preferred embodiment are further reduced by utilizing a MESFET mixer (i.e. a mixer including a metal semiconductor field effect transistor) in the low gain RF section. The MESFET mixer has almost perfect square law characteristics when operated at a low gain, and consequently the device introduces extremely small distortion into the system. In particular, the device can handle an interfering signal level at greater than +6 dBm on its output with less than 1% cross-modulation distortion and -40 dB intermodulation distortion products. Prior art television receivers, by comparison, utilize mixers comprised of bipolar or MOS transistors, diodes, or vacuum tubes, all of which generate third order products or higher odd order products to a greater degree than the disclosed MESFET mixer.
Accordingly, it is one object of the invention to provide an improved device for receiving signals in selected frequency channels from a plurality of non-overlapping frequency channels.
Another object of the invention is to provide a television receiver having improved reception.
Another object of the invention is to provide a television receiver having a low noise figure, low intermodulation distortion, and low cross-modulation distortion simultaneously.
Another object of the invention is to provide a television receiver having an RF section with a gain just large enough to achieve a desired system noise figure.
Another object of the invention is to provide a television receiver having a maximum gain before the channel selection filtering of approximately 10 dB.
Another object of the invention is to provide a television receiver having a low gain-high frequency RF-IF section.
Another object of the invention is to provide a television receiver having a low gain-high frequency RF-IF section in combination with a high gain-low frequency IF section.
Another object of the invention is to provide a television receiver suitable for integration on a single semiconductor chip.
Still another object of the invention is to provide a television receiver having a low gain MESFET mixer for frequency shifting the selected channel to a predetermined high IF.